According to the Mountain Directory West that pass southbound entails a climb of about 7.5 miles of 9% grade and then the descent involves 2 miles of 6% then anther 3 miles of 7-8%. You must then head up and down Spring Creek Pass which is not as bad with a 2 mile climb of 6% followed by several short descents of 6-7% in a rolling hills fashion.
I have driven our Phaeton all over the Rockies but would hesitate on this one myself, especially since the ride up to 50 is only an hour away. The ride up Slumgullion is long and steep and will require your coolant system to be in top notch condition. If your coach is in very good shape the ride down Slumgullion should be manageable in a very low gear and the occasional application of brakes.